A method of the general category to which the invention relates is shown in Swiss Pat. No. 577,378, in which a plate, both major surfaces of which are covered, is immersed in a strengthening fluid and in which the air contained in the plate is removed from at least one of the major surfaces of the plate. The process thus shown makes it possible to treat only one plate at a time, and it is particularly suited only for small numbers of plates or individual plates, working by hand.
German Offenlegungsschriften No. 2,715,237 shows apparatus for a similar proces and discloses means by which a more automatic method of operation is made possible.
With these devices, only one plate can be treated at a time since a uniform vacuum is not ensured when several plates are lying one on top of the other, and the air is removed from one plate surface, and this leads to uneven depth of penetration of the strengthening fluid.
Thus, in order to multiply the efficiency of such techniques, it is necessary to develop a new process.